You could get your snake started on rat pups; I just got my little female on them, and she actually seems to like them more than the mice. She was a spotty feeder on mice, but she really downs these rats. But usually, pet stores won't sell you rats until they're weaned.
I'd start off with rats that are just getting their fuzz, but whose eyes are not opened yet. They're about 2-3 inches long. I don't know about your bp, but my little girl was terrified if something was too big for her to eat, but would feed greedily on smaller mice. (and now, rats.) You could always email the company selling the rat pups, and ask them the specific length of the rats you'll be buying.